What are default parameters?

The short answer

Default parameters let you set a fallback value for function parameters when no argument is passed (or undefined is passed). They were introduced in ES6 and replace the old pattern of checking for undefined inside the function body.

How they work

Copy
function greet(name = 'World') {
  console.log(`Hello, ${name}!`);
}
greet('John'); // "Hello, John!"
greet(); // "Hello, World!"
greet(undefined); // "Hello, World!" — undefined triggers the default
greet(null); // "Hello, null!" — null does NOT trigger the default

The default value is only used when the argument is undefined or not provided. null, 0, '', and false do not trigger the default.

The old way

Before ES6, you had to check manually:

Copy
function greet(name) {
  name = name || 'World'; // breaks if name is "" or 0
  // or
  name = name !== undefined ? name : 'World';
}

The || approach is buggy because it treats "", 0, and false as missing values. Default parameters solve this correctly.

With objects

A common pattern for options objects:

Copy
function createUser({
  name = 'Anonymous',
  role = 'viewer',
} = {}) {
  return { name, role };
}
createUser({ name: 'John' }); // { name: "John", role: "viewer" }
createUser(); // { name: "Anonymous", role: "viewer" }

The = {} at the end is important — without it, calling createUser() with no arguments would throw an error because you cannot destructure undefined.

Defaults can use previous parameters

Copy
function createElement(tag, className = `${tag}-default`) {
  return { tag, className };
}
createElement('div'); // { tag: "div", className: "div-default" }
createElement('div', 'custom'); // { tag: "div", className: "custom" }

Why interviewers ask this

This tests ES6 knowledge. Interviewers want to see if you know the modern syntax and understand the edge case with null vs undefined.